not sure! (really)

'Not sure' hat viele Bedeutungen, einschl. 'ich bin mir nicht sicher'. Aber mit 'I am not sure this is a good idea' will der Sprecher höflich ausdrücken, daß er die Idee nicht gut findet.
Die Betonung liegt hier auf höflich, andernfalls würde er sagen 'this is not a good idea' oder ähnliches.

Das ist aber im Deutschen genauso. "Ich bin mir nicht sicher, dass das eine gute Idee ist." halte ich auch für eine höfliche Ablehnung.

I think the difference is made by the 'that' (and the 'such')-
not sure that this is such a good idea.... means - Forget it!
not sure whether, whether this is a good idea... is simply not being sure.
Same in German: Nicht sicher, dass...
Nicht sicher, ob....

I disagree that "not sure" is a euphemism in the general sense for "no".

In practice, however, it can be used in exactly this way, especially in the UK where understatement is used much more widely.

Thus your sense that "not sure" is a euphemism for "no" derives not from the fact that this sense of "no" adheres to the phrase itself, but rather from the fact that any phrase can take on a different meaning when used in a context where understatement is being used.

Example:

Ami: "Did you know that the Americans single-handedly invaded France in June 1944?"

Brit: "I'm not sure if that's quite how it happened, are you?" [understatement]

Translation of the Brit comment into Ami-speak:
"You've got your head so far up your ass that you need a glass belly-button to see where you're going." [hyperbolic dysphemism]

Translation into plain English:
"You're mistaken."

So, no, it's not a euphemism, but context, register, and intent color the meaning of all speech.

Peter, danke für die vermutlich treffende, aber auf jeden Fall amüsante Erläuterung. :o)

Ja, es waren Briten, auf die ich meine "Erfahrungen" bezog.

Agree with Peter that this is fairly typical Brit-speak.
My BMW neighbour once moaned to me about the British not saying what they mean: In a meeting between Rover and BMW a couple of years ago, the Germans put forward their proposals. Then the Brits got up and started off with something to the effect of: "We agree with your proposals in principle...". The German listeners were very pleased, taking the words at face value. But the British delegation then proceeded to completely reject the whole "principle" of what BMW had just suggested – the first few words had just been their polite packaging of: "Nice idea, but no thanks."

Ghol, interesting, I believe I often started an e-mail to UK people with the words "I agree with your proposals in principle" or something similar, do you believe they had probably been interpreted as a "Sorry, that's bullshit"?